A mouse device is a widely-used cursor control device of a computer system. Hereinafter, the structure of a conventional mouse device will be illustrated with reference to FIG. 1.
FIG. 1 is a schematic exploded view illustrating a conventional mouse device. First of all, the components of the conventional mouse device 1 will be illustrated. As shown in FIG. 1, the conventional mouse device 1 comprises an upper cover 11, a circuit board 12, and a lower cover 13. The upper cover 11 is divided into a left button zone 11a and a right button zone 11b with respect to a middle portion of the upper cover 11. A first switch 12a and a second switch 12b are disposed on a top surface of the circuit board 12.
For assembling the conventional mouse device 1, the circuit board 12 is firstly placed on the lower cover 13, and then the upper cover 11 is fixedly connected with the lower cover 13 from the top side of the circuit board 12. Meanwhile, the first switch 12a and the second switch 12b are disposed under the left button zone 11a and the right button zone 11b, respectively.
When the left button zone 11a or the right button zone 11b of the upper cover 11 of the mouse device 1 is depressed by the user, the first switch 12a under the left button zone 11a or the second switch 12b under the right button zone 11b is correspondingly pushed by the left button zone 11a or the right button zone 11b. Consequently, the first switch 12a or the second switch 12b is triggered to execute a computer command (e.g. a select command, a cut command, a copy command or a paste command).
From the above discussions, the upper cover 11 of the conventional mouse device 1 is divided into the left button zone 11a and the right button zone 11b, and the first switch 12a or the second switch 12b on the top surface of the circuit board 12 may be triggered by depressing the left button zone 11a or the right button zone 11b. 
However, the conventional mouse device 1 still has some drawbacks. For example, since the upper cover 11 of the conventional mouse device 1 is divided into the left button zone 11a and the right button zone 11b, the integrity of the entire outer appearance of the mouse device 1 is destroyed. Moreover, dust or other foreign matters are readily introduced into the conventional mouse device 1 through the space between the left button zone 11a and the right button zone 11b and the upper cover 11. Under this circumstance, the functions of the first switch 12a, the second switch 12b and other electronic components are possibly deteriorated, and the use life of the conventional mouse device 1 is shortened.
Therefore, there is a need of providing an improved mouse device in order to overcome the above drawbacks.